(1) Field of the Invention
This invention is related to pickup heads and more particularly to pickup heads of the Bernoulli type.
(2) Description of the Prior Art
The use of a gaseous medium such as air to support and/or lift articles is well known in the art and includes such devices as pickup heads and air track systems, for example.
In certain devices suction, i.e. vacuum or more specifically devices using a negative pressure source, is employed to lift and/or support an article. For example, a vacuum-operated pickup head for handling a filter membrane or disk is described in U.S. Pat. No. 3,608,946. However, in many applications, such as for example, the processing and handling of semiconductor wafers, the use of a vacuum device is undesirable because physical contact between the article and pickup device caused by the suction is relied upon to effect the handling. Such physical contact makes the article susceptible to contamination and/or damage.
Other devices use a positive pressure source to lift an article. In certain types of these devices the gas is simply discharged directly against the under surface of the article to be supported or lifted with sufficient force to overcome the weight of the article to be lifted. Generally, in these certain types, should the article become displaced with respect to the gas discharge, either laterally or by tilting, the article is susceptible to being placed in contacting relationship with the device per se and/or other objects. Thus, while these certain types of devices are utilized in applications such as the processing and/or handling of semiconductor wafers, the physical contact makes the article susceptible to contamination and/or damage.
The air track system described in the publication entitled "Stabilizing Jets For Air Cushion Levitated Wafer", F. Hendriks, IBM Technical Disclosure Bulletin, Vol. 18, No. 5, October 1975, page 1664, is an example of a device that employs the force of a gas discharge against the undersurface of a semiconductor wafer for levitation, lateral centering and propulsion of the wafer along the track. In addition, it provides two extra jets that are positioned on each side of the track an which direct gas discharges at an inward inclination to the top surface of the wafer to suppress certain wafer tilt, i.e. tilt about the propulsion axis. However, wafer tilt about other axes, i.e. axes which are skewed with the propulsion axis such as the lateral axis which is orthogonal to the propulsion axis, is not correctable by this device. Moreover, because this device requires interaction with both surfaces of the wafer, it is not desirable or applicable to pickup heads which react, i.e. coact, with only one surface of the article as the other surface is generally obstructed in its rest position prior to lift-off.
Other types of devices which employ a positive pressure source to lift or support an article are the so called Bernoulli types. An example of a Bernoulli type support system for supporting elongated and/or contiguous sheets of flexible metallic stock is described in U.S. Pat. No. 3,198,499 and its related divisional U.S. Pat. No. 3,319,856. While this system provides for restoring the sheet of metal if it becomes laterally displaced, it had no means for restoring the sheet should it become tilted.
Examples of Bernoulli type pickup heads are described in the following references, to wit: U.S. Pat. Nos. 3,438,668; 3,523,706 assigned to the common assignee herein; 3,539,216 and publications entitled "Wafer Pickup With Air Barrier", C. P. Hayunga; "Wafer Holder", G. O. Stenger; "Orienting Bernoulli Effect Wafer Transfer Mechanism", H. Von Burg et al appearing in the IBM Technical Disclosure Bulletin, Vol. 11, No. 2, July 1968, page 112; Vol. 13, No. 7, December 1970, page 2081; and Vol. 18, No. 6, November 1975, pages 1836, 1837, respectively. All of these devices have a main orifice, or series of orifices, coupled to a positive pressure source and are utilized to lift an article, i.e. a semiconductor wafer. Some of the devices, in addition, are provided with auxiliary ports that are also coupled to a positive pressure source so as to provide or impart a predetermined translation to the wafer in its horizontal plane, cf. U.S. Pat. Nos. 3,523,706, FIGS. 2, 2A and 3,539,216. None of the devices, however, provide means for restoring the wafer should it become tilted. It should be noted that the device described in the aforementioned publication issue of July, 1968 has a head configuration with a main central port and three symmetrical exhaust ports. These three exhaust ports are used to direct the air flow, which comes from center port, so that it is mainly exhausted in an upward and outward manner through the device thereby eliminating air flow between the periphery of the wafer and the head where the air flow might disturb an adjacent neighboring wafer. As is obvious, these exhaust ports are not coupled to a positive pressure supply and/or neither do they provide a restoring force and/or torque to the wafer should it become tilted.
For the same reasons as the vacuum type per se devices are undesirable for certain applications such as wafer processing and/or handling, so are devices which employ combinations of vacuum, i.e. suction, and the so-called Bernoulli effect. More specifically, in these devices, if the positive pressure utilized for the Bernoulli effect is diminished or stopped, the vacuum would cause the article to be placed in contact with the device. Examples of such combination type pickup heads and systems are described in the following references, to wit: U.S. Pat. Nos. 3,220,723; 3,517,958 and the publication entitled "Wafer Pickup Head", A. Leoff et al, IBM Technical Disclosure Bulletin, Vol. 14, No. 8, January 1972, pages 2311, 2312. Moreover, none of these combination type devices provide means for restoring the article should the article become tilted.